Optical nonlinearity enhancement of compositionally graded films

Abstract

We present a theoretical description for the effective linear and nonlinear optical properties of compositionally graded films consisting of nonlinear metal particles and linear dielectric particles. The volume fraction of metal particles varies along the direction perpendicular to the film. To account for the composition gradient, we resort to effective medium approximation to investigate the equivalent (local) dielectric constant and third-order nonlinear susceptibility. As a result, the effective second-rank dielectric constant tensor and fourth-rank nonlinear optical susceptibility tensor are directly determined by regarding the graded film as a multilayer one. We predict that for a power-law composition gradient p(z)=zm, the increase of m leads to large enhancement of the optical nonlinearity and hence the figure of merit in the high-frequency region. On the other hand, for a given total volume fraction, we find that the optical nonlinearity enhancement for the composition gradient case is larger than the one in non-graded case. Moreover, we can choose different graded profile to realize the appreciable optical nonlinearity enhancement. Therefore, the compositionally graded film can be served as a suitable candidate material for obtaining the large optical nonlinearity and optimal figure of merit.